This paper has the purpose of exposing the new mandatory tax disclosure mechanism adopted by the European Council of Finance Ministers, on March 13, 2018. This mechanism constitutes a new tax transparency tool for fighting against aggressive tax planning within the EU. It is a 'mandatory tax disclosure rule' that requires tax intermediaries and, in some cases, the taxpayers themselves to report to the tax authorities certain cross-border schemes that entail a certain level of fiscal risk for the public finances of the EU Member States. For their part, tax administrations must proceed to automatically and systematically exchange such information in order to allow its use in the framework of all types of actions and tax procedures for the control and management of fiscal risk. This paper critically examines the scope of the fiscal transparency measure articulated at European level, highlighting a set of issues related to its application that stress fundamental rights and guarantees of taxpayers